Neuroendocrine cells concentrate protein hormones into aggregates that form dense cores of secretory granules, a basic process whose mechanism is not known. A model is that cargo proteins diffuse to the area where secretory granules form, and specific actions occur there that change the conformation of cargo proteins so that the aggregate; the actions are not the same for all proteins. This model was developed because there is unexpected specificity in a pituitary cell line for cargo protein storage, which may reflect unrecognized specificity in the ability of the cells to aggregate proteins into dense cores. If aggregation of cargo proteins in cells is really so specific, then: 1. neuroendocrine cells will differ in their ability to aggregate cargo proteins into dense cores, and 2. the structures required for aggregation of each protein will have precise requirements, which may differ from what is required for aggregation in solution. The first two aims determine whether aggregation in intact cells is specific and determine the structures necessary. The third aim investigates another aspect of hormone production in the secretory pathway: how production of cargo protein mRNAs are regulated by feedback from that pathway. Expression of a mutant human prolactin, H27A, suppressed rat prolactin synthesis, an effect similar to dominant-negative deficiency of growth hormone and vasopressin found in people. A hypothesis that may explain the action of all these mutants is that cargo proteins that are unstable or difficult to fold result in a signal from the secretory pathway to the nucleus that reduces transcription of mRNA for all cargo proteins made by the cell. The third aim is to determine the means by which human H27A-prolactin, human H183R-growth hormone and human des32-71- growth hormone exert dominant-negative effects on hormone production. The knowledge of how hormone production and packaging is controlled in the secretory pathway will increase the ability to understand the pathogenesis of diseases such as diabetes and develop more effective treatments.